1. Field of the Invention
The present invention relates to a field emission device, and more particularly, to a field emission device in which an electrostatic quadrupole lens structure is constructed between an emitter on a cathode and an anode, improving focusing effect.
2. Description of the Related Art
Generally, a field emission device (FED) can be applied to a planar display device or a light emitting device. An FED includes a gate electrode that applies an electric field to an emitter arranged on a cathode electrode, so that the emitter emits electrons. The electrons collide with a fluorescent material coated on an anode electrode, and thus light is emitted. An FED with a double gate structure further includes a focus electrode in addition to a gate electrode which is described above.
The brightness and color purity of the FED emitting light by using electron beams emitted from a cold cathode depends not only on the material and structure of the emitter, which is the source of electrons, but also on the FED's ability to accurately focus an emitted electron beam accurately on a fluorescent material pattern to emit light. That is, to realize a high resolution display device using an FED, techniques of focusing the electron beam on the target fluorescent material pattern and not adjacent fluorescent materials are required.
Moreover, when a high voltage is applied to the anode to obtain high brightness and durability, the distance between the emitter and the anode must be increased for electrical stability. However, as the distance between the emitter and the anode increases, the electron beams are more likely to disperse. Thus, a structure that can transform an electron beam to correspond to the fluorescent material pattern and focus the beam accurately is required.
A conventional FED with a double gate structure includes an emitter emitting electrons on a cathode electrode; a gate electrode thereon which extracts electrons and has a first opening portion surrounding the emitter; and a focus electrode placed thereon which focuses the extracted electron beams and has a second opening portion having a common center with the first opening. The gate electrode is insulated from the cathode, and the focus electrode is insulated from the gate electrode.
When an electron beam emitted from one FED cannot radiate a pixel area sufficiently, a plurality of FEDs can be arranged to correspond to a pixel area.
When the focus voltage Vf is 0 V, electron beams having a circular shape reach a wide area, and as the voltage increases, the area of the electron beams decreases. However, when the focus voltage is about 50 V, a halo appears around the electron beam thus increasing the area of the electron beams.
Generally, in a display device using a FED, the fluorescent material pattern has a striped pattern with a longer vertical length than horizontal width. Since an electron beam reaching the anode has a circular shape according to the conventional double gate structure, the electron beam is likely to deviate from the width of the fluorescent material. Particularly, when two or more groups of emitters including emitters arranged in a vertical row are arranged horizontally with respect to one pixel area, the width of an electron beam increases.
Also, the optimal focusing effect is achieved at a focus voltage of −40 V. That is, to obtain sufficient focusing effect in a conventional double gate structure, the potential between a focus electrode and a gate electrode may be large, and thus an electrical breakdown may occur between the focus electrode and the gate electrode.